1. Field of the Invention
The present invention relates to an ophthalmological measuring apparatus, and more specifically to such an ophthalmological measuring apparatus using a laser beam.
2. Description of the Prior Art
A cataract, which is one of ophthalmological diseases, is a condition in which protein particles of the crystalline lens become large in size and the crystalline lens becomes opaque, and in order to discover the cataract early for proper remedy or precautionary treatment, it is necessary to measure the size or diameter of the protein particles.
A human eyeball includes various transparent bodies, such as cornea, crystalline lens and vitreous body, in which minute protein particles are floating in Brownian movement. In a normal eye the protein particles of small diameters are widely distributed, while in case of an opaque eye the protein particles of diameters larger than those in case of the normal eye are widely distributed.
In order to measure the diameter, the following method has been proposed heretofore and is now in use.
The method includes the steps of converging a laser beam on a point within the crystalline lens, detecting the intensity of scattered light scattered by protein particles being in Brownian movement when they pass across the point, obtaining by a correlator the correlation function as to the fluctuation of the scattered light intensity as the time elapses, calculating from the obtained result relaxation time of the fluctuation of the scattered light intensity, and obtaining the diffusion coefficient of the protein particles from the calculated time, thereby obtaining the diameter of the protein particles.
Such a method is carried out by a conventional apparatus as is shown in FIG. 4. As may be seen, the conventional apparatus includes a projector unit 101 for projecting a laser beam, a slit image and a spot image on a portion in the eye E to be examined, a microscope unit 102 having a beam splitter 103 for receiving the light reflected at the portion of the eye E and separating the same in two directions and incorporating a photomultiplier tube 104, a time correlator 105 connected to the photomultiplier tube 104 and an analyzer 106 connected to the time correlator 105. In order to take measurement, the projector unit 101 and the microscope unit 102 are disposed in front of the eye E to be examined.
When a measurement is taken, at first a lamp 107 is turned on, and the light emitted therefrom is converged by a lens 108 on a slit portion of a slit 109 and, after passing therethrough, it is reflected by a mirror 110, passes through a half mirror 111 and goes into a lens 112, reflected by a moving mirror 113 and goes along the optical axis X.sub.1 to form a slit image on a portion in the eye E including a measuring point P by a projecting lens 114.
When a lamp 115 is turned on, the light therefrom illuminates a pin hole 116 and forms a spot image or pin hole image on the measuring point P through the lens 112, the moving mirror 113 and the projecting lens 114 which are optically in conjugate relation to each other.
While the slit image or the spot image is observed through an eyepiece 117 on an observing system optical axis X.sub.3 extending at a predetermined angle to the optical axis X.sub.1, the projecting angle to the eye E to be examined, that is the optical axis X.sub.1, and the optimum observing angle to the optical axis X.sub.1, that is the orientation of the optical axis X.sub.3 in relation thereto are adjusted so that observation and measurement at the measuring point P can be made in the optimum condition.
Then, the moving mirror 113 is moved in the direction of an arrow A to be retracted from the optical path of the optical axis X.sub.1, and a laser tube (not shown) is actuated to emit a laser beam L.
The laser beam L, once converged by a condenser lens 118, is diverged and goes into a collimating lens 119 in which the laser beam L becomes substantially parallel. The parallel beam reaches the projecting lens 114 by which it is converged and directed to the measuring point P.
The laser beam scattered at the the measurement point P partly goes through the beam splitter 103 and then through an objective lens 120 and an imaging lens 121 to be observed by the eyepiece 117. At the same time, a part of the laser beam scattered is reflected by the reflecting surface 103a of the beam splitter 103 and directed to the photomultiplier tube 104 to be measured by a measuring device connected thereto.
Thus, in the above mentioned conventional apparatus in which the slit illuminating system and the spot illuminating system are superposed on the optical path of the laser beam for measurement, preliminary observation and adjustment of the measuring location may be easily made and its operation may be quickly accomplished. However, as the optical axis in which the laser beam is projected and the optical axis of the microscope are arranged at a certain angle to each other, they are refracted differently on the surface of the eyeball, causing a slight locational difference between the convergent point of the laser beam and the focal point of the microscope. Therefore, at some depths in the eyeball the convergent point of the laser beam does not conform to the location at which the scattered light is measured. Furthermore, the conventional apparatus is disadvantageous in that the area of measurement cannot be assured.